Polyallylamine is a polymer of allylamine and comprises a long carbon chain with every alternate carbon bearing a pendant amino group. Each amino group is suspended from the chain by a methylene group and the polymeric structure can be represented as below.

Polyallylamine can be crosslinked with itself (to form loops) or to other polyallylamine chains (to form ladder-like structures) or with other compounds through the amino groups. There are innumerable crosslinking agents known in the art ranging from the simple to the bulky, exemplary of which are 1,2 dichloroethane and sorbitol poly-glycidal ether. Depending on the requirements of the crosslinked polyallylamine the distance between the crosslinked chains can be manipulated effectively by judicious choice of the crosslinking agent.
The amino groups of polyallylamine also allow for further modification and functionalization. The combination of these functionalized amino groups and suitable crosslinking agents leads to endless possibilities and varieties of polyallylamine polymers. It is possibly this permutation that lends itself to the versatility of polyallylamine polymers for they are useful in fields as wide-ranging as waste-water treatment (Journal of Applied Polymer Science, 2001, 80, 2073) and pharmaceuticals. Polyallylamine linked with glucosyl or galactosyl moieties has been disclosed for the treatment of Diabetes, Metabolic syndrome and obesity in WO2007/070135. It has been linked to cyclodextrins for use as drug delivery vectors (Nature Reviews: Drug Discovery 2004, 3, 1023) and for the preparation of polymeric colloid nanoparticles—WO2005/015160. Polyallylamine crosslinked using epichlorohydrin has been approved by the U.S. FDA for therapeutic use in the treatment of chronic renal failure—Sevelamer, and for the reduction of elevated LDL cholesterol—Colesevelam. It also finds its uses in other fields like electrochemistry and as resins.
In 1985, the Japanese company, Nitto Boseki patented an industrial process to polymerize allylamine using Azo-compounds—U.S. Pat. No. 4,504,640 and the crosslinking of polyallylamine—U.S. Pat. No. 4,605,701. Allylamine can also be polymerized using tetraflurorohydrazine—N2F4 (U.S. Pat. No. 3,062,798) and with hydrogen peroxide in the presence of a multivalent metal ion (U.S. Pat. No. 4,927,896).
According to the crosslinking process of U.S. Pat. No. 5,496,545, an aqueous solution of polyallylamine hydrochloride is neutralized using sodium hydroxide and then the crosslinking agent, for example epichlorohydrin, is added to it. Within 15 minutes of the addition of epichlorohydrin the reaction mixture gels. The gel is cured for about eighteen hours at room temperature and then put in a blender to get coarse particles. The formation of a gel leads to handling difficulties and always necessitates blending to get the desired product.
When polyallylamine is crosslinked as described in U.S. Pat. No. 4,605,701, the polymer is obtained not as a gel, but as small globules. According to this process the crosslinking agent is added directly to the partially neutralized aqueous solution of polyallylamine hydrochloride. This process precludes gel formation by using a dispersing agent in the crosslinking step and stirring which results in the formation of small-globular polymer. However when this process is carried out on an industrial scale, the formation of a gel cannot be avoided entirely.
U.S. Pat. No. 6,525,113 describes a process for the preparation of crosslinked polyallylamine hydrochloride in which the swelling of the polymer is controlled. According to this process an aqueous solution of polyallylamine is first neutralized by using an alkoxide or a hydroxide. To this reaction mixture a water-miscible organic solvent is added. This solvent is said to displace the water out of the polyallylamine particles. The crosslinking agent is then added to the reaction mixture and the suspension that is formed after crosslinking is filtered to recover the product.
US 2006/258812 discloses a process for the preparation of a crosslinked polyallylamine polymer that has a particle size between 60 and 100 mesh. The process consists of adding the cross-linking agent to the aqueous solution of partially neutralized polyallylamine hydrochloride and dispersing the whole in an organic solvent that contains a surfactant. After a period of heating with concurrent stirring, the gel particles are isolated by filtration.